Navigation satellite systems (NSS) include both global navigation satellite systems (GNSS) and regional navigation satellite systems (RNSS), such as the Global Positioning System (GPS) (United States), GLONASS (Russia), Galileo (Europe), BeiDou (China), and the Indian Regional Navigational Satellite System (IRNSS) (systems in use or in development). A NSS typically uses a plurality of satellites orbiting the Earth. The plurality of satellites forms a constellation of satellites. A NSS receiver detects a code modulated on an electromagnetic signal broadcast by a satellite. The code is also called a ranging code. Code detection includes comparing the bit sequence modulated on the broadcasted signal with a receiver-side version of the code to be detected. Based on the detection of the time of arrival of the code for each of a series of the satellites, the NSS receiver estimates its position. Positioning includes, but is not limited to, geolocation, i.e. the positioning on the surface of the Earth.
An overview of GPS, GLONASS and Galileo is provided for instance in sections 9, 10 and 11 of Hofmann-Wellenhof B., et al., GNSS, Global Navigation Satellite Systems, GPS, GLONASS, Galileo, & more, Springer-Verlag Wien, 2008, (hereinafter referred to as “reference [1]”).
Positioning using NSS signal codes provides a limited accuracy, notably due to the distortion the code is subject to upon transmission through the atmosphere. For instance, the GPS includes the transmission of a coarse/acquisition (C/A) code at 1575.45 MHz, the so-called L1 frequency. This code is freely available to the public, in comparison to the Precise (P) code, which is reserved for military applications. The accuracy of code-based positioning using the GPS C/A code is approximately 15 meters, when taking into account both the electronic uncertainty associated with the detection of the C/A code (electronic detection of the time of arrival of the pseudorandom code) and other errors including those caused by ionospheric and tropospheric effects, ephemeris errors, satellite clock errors and multipath propagation.
An alternative to positioning based on the detection of a code is positioning based on carrier phase measurements. In this alternative approach or additional approach (ranging codes and carrier phases can be used together for positioning), the carrier phase of the NSS signal transmitted from the NSS satellite is detected, not (or not only) the code modulated on the signal transmitted from the satellite.
The approach based on carrier phase measurements has the potential to provide much greater position precision, i.e. up to centimeter-level or even millimeter-level precision, compared to the code-based approach. The reason may be intuitively understood as follows. The code, such as the GPS C/A code on the L1 band, is much longer than one cycle of the carrier on which the code is modulated. The position resolution may therefore be viewed as greater for carrier phase detection than for code detection.
However, in the process of estimating the position based on carrier phase measurements, the carrier phases are ambiguous by an unknown number of cycles. The phase of a received signal can be determined, but the number of cycles cannot be directly determined in an unambiguous manner. This is the so-called “integer ambiguity problem”, “integer ambiguity resolution problem” or “phase ambiguity resolution problem”, which may be solved to yield the so-called fixed solution.
GNSS observation equations for code observations and for carrier phase observations are for instance provided in reference [1], section 5. An introduction to the GNSS integer ambiguity resolution problem, and its conventional solutions, is provided in reference [1], section 7.2. The skilled person will recognize that the same or similar principles apply to RNSS systems.
In order to improve the positioning process at the receivers, such as to improve the performance of position determination systems, some systems involve sending correction information to the receivers. Such correction information may generally be seen as comprising information useful to correct NSS observations made by a receiver. For example, the correction information may represent data relating to the NSS system that may be taken into account and used to improve the estimation of the receiver position. The correction information may comprise correction data relating to NSS satellites, such as, but not limited to, accurate orbital data and accurate satellite clock data to improve the positioning solution.
The correction information may be computed or prepared by a network of reference receivers with precisely known positions in a global reference frame (i.e., coordinate system). A typically world-wide network of reference receivers is used for GNSS systems, whereas a regional network of reference receivers is typically sufficient for RNSS systems. The data from the reference receivers is transmitted for example over the internet to a processing centre, where the data is collected, synchronized and processed. During the data processing, a variety of products can be generated, including e.g. satellite orbits, satellite clock errors, GNSS (or RNSS) measurement biases, and atmospheric effects. The products (or corrections) are then sent to the rover receivers on the field. The transmission to the rover can take place in many different forms, of which the most commonly used are the internet and satellite links. For a descriptive example of a global GNSS positioning correction service see e.g. WO 2011/034616 A2 (applicant reference: TNL A-2585PCT).
There is a constant need for improving the implementation of positioning systems based notably on GNSS (or RNSS) carrier phase measurements, to obtain a precise estimation of the receiver position.